This invention relates generally to process control, and, more specifically, to the processing of a layer through a surface thereof under conditions where either the layer or the processing, or both, is uneven across the surface.
There are many situations where such processing takes place. One class of such situations occurs in the manufacturing of flat panel liquid crystal displays and integrated circuits on semiconductor wafers. In each, several processing steps include forming a layer of material and subsequently removing either all or a portion of the layer according to a pattern. Uniformity of the forming and removing operations is important, primarily because of the extremely small variations that are allowed in the finished products. As part of this process, it must be accurately determined when just enough of the layer has been removed; ie, to detect an endpoint of the removal operation. The endpoint determination is then used to monitor the progress of the process and/or to control the process, such as by automatically terminating the specific processing operation being monitored.
At several stages in the manufacture of display panels or circuits on a semiconductor wafer, a mask is formed of photoresist material. The resulting mask is used to limit processing of a layer covered by the mask to a patterned area. The mask is formed by exposing the photoresist layer to light in the desired pattern, followed by developing the photoresist layer through application of a developer solution to it. With the usual photoresist material, the exposed regions are removed during the development process to expose the layer below. The time at which the underlying layer first becomes exposed by removal of photoresist material is termed the "breakthrough" or "endpoint." The development process is allowed to continue for a period of time after breakthrough is first detected, the end of that period of time being the end of the development process, termed its "process end" (or "step end" if there is more than one step in the overall process).
Because of various processing and environmental variations that exist among semiconductor wafers, the development process is monitored in order to determine when breakthrough occurs. A beam of light having a finite bandwidth is directed against the photoresist layer of one wafer and a reflected or transmitted light signal is then detected in, the resulting electrical signal processed to determine when breakthrough occurs. In one form, light reflected from both the top and bottom surfaces of the substantially transparent photoresist layer interferes at, and is detected by, a photodetector. As a portion of the photoresist layer is removed during the development process, the detected intensity of the reflected light cycles between one or more maxima and minima as the material removal alters the relative phase between the two interfering beams. At breakthrough, however, this signal variation ends, a condition which is detected by analyzing the photodetector output signal. Development is then usually allowed to proceed for a fixed or calculated time after detection of breakthrough, at which point the development is terminated by rinsing away the development solution or by some other means.
Wet etching processes, wherein substantially transparent material layers other than photoresist material are etched away, also use a similar breakthrough detection process. Where the layer being etched is opaque, the photodetector signal either suddenly rises or falls when breakthrough occurs. In the case of a dry etching process, wherein material is removed by bombardment in a plasma chamber, a species within the plasma is monitored by detecting the emission of light in a limited wavelength band that shows material is being removed from the layer being etched. The detected signal in a plasma application drops off considerably at breakthrough. These sudden changes that occur at breakthrough are also detectable.
It is an object of the present invention to provide improved techniques of monitoring and controlling these and characteristically similar types of processing operations.
It is another object of the invention to provide a technique of monitoring and controlling the uniformity of such processing across surface areas being treated.
It is a special object of the present invention to provide an improved method of manufacturing with an improved yield large flat liquid crystal display panels.